Identification of a new family of protein phosphatase 2A regulatory subunits

Beta-adrenergic receptor agonists and cyclic nucleotide phosphodiesterase inhibitors: shifting the focus from inotropy to cyclic adenosine monophosphate

Clinical trials of beta-adrenergic receptor agonists and cyclic nucleotide phosphodiesterase inhibitors in heart failure have demonstrated a reduction in survival in treated patients despite initial inotropic responses. These findings have led many to infer that activation of the mechanisms through which contractility is increased has deleterious effects on failing myocardium. It should be remembered, however, that these agents act proximately by raising intracellular cyclic adenosine monophosphate (cAMP) content and stimulating protein phosphorylation by cAMP-dependent protein kinase, and that the proteins whose phosphorylation contributes to the inotropic responses may be different from the proteins whose phosphorylation contributes to the reduction in survival. Evidence in support of the latter interpretation is presented, and potential therapeutic approaches through which the phosphorylation of different proteins might be selectively affected are considered.

Developmental expression of protein phosphatase 2A in the kidney

Although a number of growth and transcription factors are known to regulate renal growth and development, the signal transduction molecules necessary to mediate these developmental signals are relatively unknown. Therefore, the activity and mRNA and protein expression of the signal transduction molecule protein phosphatase 2A (PP2A) were examined during rat kidney development. Northern analysis of total kidney RNA or Western analysis of kidney protein homogenates from embryonic day 15 to 90-d-old adults demonstrated developmental regulation of the catalytic, major 55-kD B regulatory subunit and A structural subunit with the highest levels of expression in late embryonic and newborn kidneys. Similarly, okadaic acid-inhibitable phosphatase enzyme activity was highest in the embryonic and newborn kidney. To map cell-specific expression of PP2A in the developing kidney, in situ hybridization with a catalytic subunit digoxigenin-labeled cRNA was performed on embryonic day 20 and newborn kidneys. PP2A was found predominately in the nephrogenic cortex and particularly in the developing glomeruli and non-brush border tubules in the embryonic day 20 and newborn kidneys. Similarly, immunocytochemistry with a specific PP2A catalytic subunit polyclonal anti-peptide antibody demonstrated catalytic subunit protein particularly concentrated in the podocytes of glomeruli in the newborn kidney. In the adult kidney, PP2A protein was no longer detectable except in the nuclei of distal tubular cells. Therefore, the developmental regulation of PP2A activity and protein during kidney development and its mapping to the nephrogenic cortex, developing glomeruli, and tubules suggests a role for PP2A in the regulation of nephron growth and differentiation.

Regulation of protein phosphorylation in astrocyte cultures by external calcium ions: specific effects on the phosphorylation of glial fibrillary acidic protein (GFAP), vimentin and heat shock protein 27 (HSP27)

The effect of external Ca2+ ([Ca2+]e) on the incorporation of [32P] into total protein, cytoskeletal proteins and the heat shock protein HSP27, was studied in primary cultures of astrocytes from the rat hippocampus. Zero [Ca2+]e increased total 32P-incorporation into astrocyte protein and when this was normalized to 100%, incorporation was significantly increased into glial fibrillary acidic protein (GFAP), vimentin (VIM) and HSP27. The difference in total 32P-incorporation between zero [Ca2+]e and 1 mM [Ca2+]e was reversed by incubation of the cells with the protein phosphatase inhibitor okadaic acid in the range 1-10 nM; higher concentrations of okadaic acid (50-100 nM) further increased total 32P-incorporation. In zero [Ca2+]e the non-specific channel blocker Co2+ (1 mM) decreased total 32P-incorporation by approximately 30%. The results were compared with a previous study [S.T. Wofchuk, R. Rodnight, Age-dependent changes in the regulation by external calcium ions of the phosphorylation of glial fibrillary acidic protein in slices of rat hippocampus, Dev. Brain Res. 85 (1995) 181-186] in which it was shown that in immature hippocampal slices zero [Ca2+]e compared with 1 mM [Ca2+]e increased 32P-incorporation into GFAP without changing total incorporation. The difference between the results for total 32P-incorporation obtained in cultured astrocytes and immature brain tissue was found to be related to the concentration of [Ca2+]e in the medium since in slices concentrations of [Ca2+]e higher than 1 mM progressively decreased total incorporation. The difference may reflect a higher Ca2+-permeability of the plasma membrane in cultured astrocytes and/or to the complex structure of the slice tissue. In two-dimensional electrophoresis HSP27, in contrast to GFAP and VIM, was separated into 3 immunodetectable isoforms only two of which were normally phosphorylated. After labelling in the presence of okadaic acid both immunodetectable and phosphorylated HSP27 focussed as a single polypeptide. Phorbol dibutyrate (1 microM) and zero [Ca2+]e stimulated the phosphorylation of both isoforms, but in the case of zero [Ca2+]e the effect on the more acidic isoform was proportionally greater.

Brain protein serine/threonine phosphatases

All of the known protein serine/threonine phosphatases are expressed in the brain. These enzymes participate in a variety of signaling pathways that modulate neuronal activity. The multifunctional activity of many serine/threonine phosphatases is achieved through their association with targeting proteins. Identification and analysis of targeting molecules has led to new insights into the functions of protein phosphatases in neuronal signaling. The recent use of transgenic mice has also increased our understanding of the physiological roles of these enzymes in the brain.

Vimentin dephosphorylation by protein phosphatase 2A is modulated by the targeting subunit B55

The intermediate filament protein vimentin is a major phosphoprotein in mammalian fibroblasts, and reversible phosphorylation plays a key role in its dynamic rearrangement. Selective inhibition of type 2A but not type 1 protein phosphatases led to hyperphosphorylation and concomitant disassembly of vimentin, characterized by a collapse into bundles around the nucleus. We have analyzed the potential role of one of the major protein phosphatase 2A (PP2A) regulatory subunits, B55, in vimentin dephosphorylation. In mammalian fibroblasts, B55 protein was distributed ubiquitously throughout the cytoplasm with a fraction associated to vimentin. Specific depletion of B55 in living cells by antisense B55 RNA was accompanied by disassembly and increased phosphorylation of vimentin, as when type 2A phosphatases were inhibited using okadaic acid. The presence of B55 was a prerequisite for PP2A to efficiently dephosphorylate vimentin in vitro or to induce filament reassembly in situ. Both biochemical fractionation and immunofluorescence analysis of detergent-extracted cells revealed that fractions of PP2Ac, PR65, and B55 were tightly associated with vimentin. Furthermore, vimentin-associated PP2A catalytic subunit was displaced in B55-depleted cells. Taken together these data show that, in mammalian fibroblasts, the intermediate filament protein vimentin is dephosphorylated by PP2A, an event targeted by B55.

Regulation of protein phosphatase 2A activity by heat shock transcription factor 2

Heat shock transcription factor (HSF) mediates the stress-induced expression of heat shock protein genes (hsp). However, HSF is required for normal cell function even in the absence of stress and is important for cell cycle progression, but the mechanism that mediates these effects of HSF is unknown. Here, it is shown that a member of the HSF family, HSF2, interacts with the PR65 (A) subunit of protein phosphatase 2A (PP2A). HSF2 binding to PR65 blocks its interaction with the catalytic subunit, due to competition between HSF2 and catalytic subunit for the same binding site in PR65. In addition, overexpression of HSF2 stimulates PP2A activity in cells, indicating the relevance of HSF2 as a regulator of PP2A in vivo. These results identify HSF2 as a dual function protein, capable of regulating both hsp expression and PP2A activity. This could function as a mechanism by which hsp expression is integrated with the control of cell division or other PP2A-regulated pathways.

Identification of progesterone-dependent messenger ribonucleic acid regulatory patterns in the rhesus monkey endometrium by differential-display reverse transcription-polymerase chain reaction

We used differential-display reverse transcription-polymerase chain reaction (DDRT-PCR) to identify different patterns of progesterone (P4)-dependent gene regulation in rhesus monkey endometria. Complementary DNA populations representing the proliferative phase (estrogen dominant, EcDNA) and an inadequate secretory phase (low level of P4, IcDNA) were compared with a cDNA population representing an adequate secretory phase (normal level of P4, PcDNA). We were able to distinguish four different levels of mRNA regulation: 1) up-regulation by P4 during an adequate secretory phase, 2) autologous down-regulation (IcDNA versus PcDNA), 3) lower abundance in IcDNA compared to PcDNA, and 4) P4-dependent inhibition of EcDNA gene expression. We isolated and sequenced 16 fragments representing these different levels of P4 regulation. The sequence of three fragments that were autologously down-regulated (I1, I2, I4) matched previously entered GenBank mRNAs: I1 encodes serine/threonine protein phosphatase A; I2 encodes oxobutanoate dehydrogenase E1b-beta; and I4 encodes line-1 reverse transcriptase homologue. Six other fragments exhibited homology to uncharacterized expressed sequence tags, sequence site tags, and cosmid clones. The remaining seven fragments exhibited no significant homology to GenBank entries at this time. The various patterns of P4-dependent gene regulation identified in the present study are likely to play roles in the temporal orchestration of events that lead to proper maturation of the endometrium.

Regulation of beta-catenin signaling by the B56 subunit of protein phosphatase 2A

Dysregulation of Wnt-beta-catenin signaling disrupts axis formation in vertebrate embryos and underlies multiple human malignancies. The adenomatous polyposis coli (APC) protein, axin, and glycogen synthase kinase 3beta form a Wnt-regulated signaling complex that mediates the phosphorylation-dependent degradation of beta-catenin. A protein phosphatase 2A (PP2A) regulatory subunit, B56, interacted with APC in the yeast two-hybrid system. Expression of B56 reduced the abundance of beta-catenin and inhibited transcription of beta-catenin target genes in mammalian cells and Xenopus embryo explants. The B56-dependent decrease in beta-catenin was blocked by oncogenic mutations in beta-catenin or APC, and by proteasome inhibitors. B56 may direct PP2A to dephosphorylate specific components of the APC-dependent signaling complex and thereby inhibit Wnt signaling.

Direct metal analyses of Mn2+-dependent and -independent protein phosphatase 2A from human erythrocytes detect zinc and iron only in the Mn2+-independent one

A Mn2+-dependent protein phosphatase 2A which is composed of a 34 kDa catalytic C' subunit and a 63 kDa regulatory A' subunit, was purified from human erythrocyte cytosol. C' and A' produced V8- and papain-peptide maps identical to those of the 34 kDa catalytic C and the 63 kDa regulatory A subunits of the Mn2+-independent conventional protein phosphatase in human erythrocyte cytosol, respectively. Reconstitution of C'A and CA' revealed that the metal dependency resided in C' and not in A'. In CA, 0.87 +/- 0.12 mol zinc and 0.35 +/- 0.18 mol iron per mol enzyme were detected by atomic absorption spectrophotometry, but manganese, magnesium and cobalt were not detected. None of these metals was detected in C'A'. Pre-incubation of C' with ZnCl2 and FeCl2, but not FeCl3, synergistically stimulated the Mn2+-independent protein phosphatase activity. The protein phosphatase activity of C was unaffected by the same zinc and/or iron treatment. These results suggest that C is a Zn2+- and Fe2+-metalloenzyme and that C' is the apoenzyme.

Alpha4 protein as a common regulator of type 2A-related serine/threonine protein phosphatases

The catalytic activity of the C subunit of serine/threonine phosphatase 2A is regulated by the association with A (PR65) and B subunits. It has been reported that the alpha4 protein, a yeast homolog of the Tap42 protein, binds the C subunit of serine/threonine phosphatase 2A and protein phosphatase 2A-related protein phosphatases such as protein phosphatase 4 and protein phosphatase 6. In the present study, we showed that alpha4 binds these three phosphatases and the association of alpha4 reduces the activities of these phosphatases in vitro. In contrast, PR65 binds to the C subunit of serine/threonine phosphatase 2A but not to protein phosphatase 4 and protein phosphatase 6. These results suggest that the alpha4 protein is a common regulator of the C subunit of serine/threonine phosphatase 2A and protein phosphatase 2A-related protein phosphatases.

Protein phosphatase 2A: who shall regulate the regulator?

Protein phosphatases are responsible for keeping the signaling output of stimulus-activated protein kinases in check; but protein phosphatases are also themselves targets and conveyors of biological signals. Among the major serine/threonine phosphatases, protein phosphatase 2A (PP2A) appears to play a privileged role in the regulation of cell growth and division. How PP2A is regulated is an intriguing question. This review will focus on the role of local protein-protein interactions in PP2A control. Work from a number of laboratories has shown that the catalytic activity, substrate specificity, and subcellular targeting of PP2A are regulated by a remarkably diverse range of regulatory subunits and enzyme inhibitors. On the pathological side, DNA tumor viruses subvert PP2A function by producing proteins that compete with specific regulatory subunits. By interfering with PP2A, these viral proteins can elicit changes in the activity of specific signal transduction pathways, such as the mitogen-activated protein kinase cascade. Recent data indicate that besides classical holoenzyme forms, a fraction of PP2A molecules are associated with novel partners implicated in signal transduction. PP2A biochemically and genetically interacts with the Tap42/alpha4 protein, which is part of a rapamycin-sensitive pathway that connects extracellular stimuli to the initiation of mRNA translation. PP2A also binds to CK2alpha, the catalytic subunit of CK2 (formerly casein kinase 2), and binding is sensitive to mitogenic signaling. The potent effect of quantitatively minor PP2A partners might be explained by a general requirement for docking interactions with substrates under intracellular conditions.

Molecular characterization of the B' regulatory subunit gene family of Arabidopsis protein phosphatase 2A

Type 2A serine/threonine protein phosphatases (PP2A) have been implicated as important mediators of a diverse array of reversible protein phosphorylation events in plants. We have identified a novel Arabidopsis gene (AtB' delta) which encodes a 55-kDa B' type regulatory subunit of PP2A. The protein encoded by this gene is 57-63% identical and 69-74% similar to the previously identified AtB' genes. The AtB' delta gene appears to be expressed in all Arabidopsis organs indicating its protein product has a basic housekeeping function in plant cells. Unlike certain mRNAs derived from the AtB' gamma gene, AtB' delta mRNAs do not fluctuate significantly in response to heat stress. Further analysis of cDNA sequences derived from the AtB' genes identified an alternatively spliced cDNA derived from AtB' gamma. This cDNA differs from the previously identified AtB' gamma cDNA by the absence of a 133-bp region in its 5' untranslated region. The missing 133-bp region appears to constitute an unspliced intron and its presence in the AtB' gamma gene was confirmed by PCR using Arabidopsis genomic DNA as a template. AtB' gamma mRNA containing the 133-bp intron accumulate in all Arabidopsis organs and their levels fluctuate differentially in response to heat stress. The 133-bp insert contains two short open reading frames and hence might serve as a translational control mechanism affecting AtB' gamma protein synthesis. Finally we show, using both the yeast two hybrid system and in vitro binding assays, that the B' subunit of Arabidopsis PP2A is able to associate with other PP2A subunits, supporting the notion that the B' protein serves as a regulator of PP2A activity in plants.

Functional interaction between a novel protein phosphatase 2A regulatory subunit, PR59, and the retinoblastoma-related p107 protein

The proteins of the retinoblastoma family are potent inhibitors of cell cycle progression. It is well documented that their growth-inhibitory activity can be abolished by phosphorylation on serine and threonine residues by cyclin dependent kinases. In contrast, very little is known about the dephosphorylation of retinoblastoma-family proteins. We report here the isolation, by virtue of its ability to associate with p107, of a novel Protein Phosphatase 2A (PP2A) regulatory subunit, named PR59. PR59 shares sequence homology with a known regulatory subunit of PP2A, PR72, but differs from PR72 in its expression pattern and its functional properties. We show that PR59 co-immunoprecipitates with the PP2A catalytic subunit, indicating that PR59 is a genuine component of PP2A holo-enzymes. In vivo, PR59 associates specifically with p107, but not with pRb. Elevated expression of PR59 results in dephosphorylation of p107, but not of pRb, and inhibits cell proliferation by causing cells to accumulate in G1. These data support a model in which the distinct PP2A regulatory subunits act to target the PP2A catalytic subunit to specific substrates and suggest a role for PP2A in regulation of p107.

The structure of the protein phosphatase 2A PR65/A subunit reveals the conformation of its 15 tandemly repeated HEAT motifs

The PR65/A subunit of protein phosphatase 2A serves as a scaffolding molecule to coordinate the assembly of the catalytic subunit and a variable regulatory B subunit, generating functionally diverse heterotrimers. Mutations of the beta isoform of PR65 are associated with lung and colon tumors. The crystal structure of the PR65/Aalpha subunit, at 2.3 A resolution, reveals the conformation of its 15 tandemly repeated HEAT sequences, degenerate motifs of approximately 39 amino acids present in a variety of proteins, including huntingtin and importin beta. Individual motifs are composed of a pair of antiparallel alpha helices that assemble in a mainly linear, repetitive fashion to form an elongated molecule characterized by a double layer of alpha helices. Left-handed rotations at three interrepeat interfaces generate a novel left-hand superhelical conformation. The protein interaction interface is formed from the intrarepeat turns that are aligned to form a continuous ridge.

Binding specificity of protein phosphatase 2A core enzyme for regulatory B subunits and T antigens

The core enzyme of protein phosphatase 2A is composed of a regulatory subunit A and a catalytic subunit C. It is controlled by three types of regulatory B subunits (B, B', and B") and by tumor (T) antigens, which are unrelated by sequence but bind to overlapping regions on the A subunit. To find out whether the different B subunits and T antigens bind to identical or distinct amino acids of the A subunit, mutants were generated and their abilities to bind B subunits and T antigens were tested. We found that some amino acids are involved in the binding of all types of B subunits, whereas others are specifically involved in the binding of one or two types of B subunits. T-antigen-binding specificity does not correlate with that of a particular type of B subunit.

Protein phosphatase 2A is required for the initiation of chromosomal DNA replication

Protein phosphatase 2A (PP2A) is an abundant, multifunctional serine/threonine-specific phosphatase that stimulates simian virus 40 DNA replication. The question as to whether chromosomal DNA replication also depends on PP2A was addressed by using a cell-free replication system derived from Xenopus laevis eggs. Immunodepletion of PP2A from Xenopus egg extract resulted in strong inhibition of DNA replication. PP2A was required for the initiation of replication but not for the elongation of previously engaged replication forks. Therefore, the initiation of chromosomal DNA replication depends not only on phosphorylation by protein kinases but also on dephosphorylation by PP2A.

Physical and functional interactions between type I transforming growth factor beta receptors and Balpha, a WD-40 repeat subunit of phosphatase 2A

We have previously shown that a WD-40 repeat protein, TRIP-1, associates with the type II transforming growth factor beta (TGF-beta) receptor. In this report, we show that another WD-40 repeat protein, the Balpha subunit of protein phosphatase 2A, associates with the cytoplasmic domain of type I TGF-beta receptors. This association depends on the kinase activity of the type I receptor, is increased by coexpression of the type II receptor, which is known to phosphorylate and activate the type I receptor, and allows the type I receptor to phosphorylate Balpha. Furthermore, Balpha enhances the growth inhibition activity of TGF-beta in a receptor-dependent manner. Because Balpha has been characterized as a regulator of phosphatase 2A activity, our observations suggest possible functional interactions between the TGF-beta receptor complex and the regulation of protein phosphatase 2A.

Regulation of protein phosphatase 2A catalytic activity by alpha4 protein and its yeast homolog Tap42

Recent studies have revealed that the alpha4 protein, a mammalian homolog of yeast Tap42, is associated with the protein phosphatase 2A catalytic subunit (PP2A-C), however, effects of the association of alpha4 with PP2A-C on its phosphatase activity have not been examined, especially using physiologically relevant substrates in the signaling pathway of mTOR (the mammalian target of rapamycin) protein. Here, we report how this association affects the enzymatic activity of PP2A-C using the recombinant eIF-4E binding protein (4E-BP1) phosphorylated by immunoprecipitated mTOR as a substrate. PP2A-C dephosphorylated 4E-BP1 in vitro. The association of alpha4 and Tap42 with PP2A-C inhibited the phosphatase activity toward 4E-BP1. Rapamycin treatment, however, neither induced restoration of the phosphatase activity of PP2A-C nor caused dissociation of alpha4 and Tap42 from PP2A-C. Our study is the first report to reveal a potential regulatory role of alpha4 and Tap42 to inibit the phosphatase activity of PP2A-C toward the physiologically relevant substrate in the mTOR signaling.

Linkage mapping of the Bra, Brb and Brg genes for rat protein phosphatase 2A 55 kDa B-regulatory subunit isotypes

We previously identified the rat Bra, Brb and Brg genes, which encode alpha, beta and gamma isotypes of the 55 kDa B-regulatory subunit of protein phosphatase 2A. Polymerase chain reaction-single strand conformation polymorphism analysis in the present study identified polymorphisms in Bra, Brb and Brg between the ACI and BUF, ZI and TM, and BN and WTC strains, respectively. Linkage analysis using mapping panels composed of F2 or back-crosses of these strains allowed Bra, Brb and Brg to be assigned to chromosomes 15, 18 and 14, respectively. Furthermore, it was revealed that Bra is located close to the Rb1 locus. Using polymorphism in Bra, loss of heterozygosity (LOH) was analyzed for rat mammary tumors induced in (SD x F344) F1 female rats by a food-borne carcinogen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, and a typical mammary carcinogen, 7,12-dimethylbenz[a]anthracene. No LOH was detected at the Bra locus.

Alpha 4 associates with protein phosphatases 2A, 4, and 6

Protein phosphatases participate in the regulation of a variety of cellular processes. Control of their enzymatic activity and specificity is made possible largely by an array of regulatory subunits. Novel serine/threonine phosphatases--PP4 and PP6 in human cells--have been discovered recently, for which regulatory subunits are yet to be identified. We report here the identification of a potential regulatory subunit of these phosphatases. Using the yeast two-hybrid system, we have found that alpha 4, a previously identified phosphoprotein, associates constitutively with the catalytic subunits of PP4, PP6, and both isoforms of PP2A. These interactions have been confirmed by direct binding and do not require phosphorylation of alpha 4, although it is unclear whether alpha 4 phosphorylation has any effect on its association with the phosphatases. The binding activity appears to reside in the N-terminal 50 amino acids of the phosphatases, consistent with a previous observation that the first 55 residues of PPV, a Drosophila homolog of PP6, may harbor the element for regulation. alpha 4 shares 37% sequence homology with Tap42, an S. cerevisiae protein that has been reported to associate with PP2A and Sit4 (yeast homolog of PP6) and comprises a regulatory component in the rapamycin-sensitive Tor signalling pathway. By analogy, alpha 4 and its associated phosphatases may participate in the mammalian rapamycin-sensitive pathway mediated by FRAP.

Increasing the ratio of PP2A core enzyme to holoenzyme inhibits Tat-stimulated HIV-1 transcription and virus production

We demonstrated previously that PP2A exists in many cell types as two abundant forms: (1) holoenzyme composed of two regulatory subunits, A and B, and a catalytic subunit C; and (2) core enzyme consisting of the A and C subunits. These two forms have different substrate specificities. Since published data suggested that HIV-1 transcription may be regulated by a cellular protein phosphatase, it was of interest to determine whether changing the ratio between PP2A core and holoenzyme affects HIV-1 gene expression. This question was addressed by expression in COS cells of an N-terminal mutant of the A subunit, A delta 5, which binds the C but not the B subunit. This resulted in an increase in the amount of core enzyme and a decrease in the amount of holoenzyme concomitant with the expected change in phosphatase activity. Tat-stimulated transcription from the HIV-1 LTR was inhibited 5-fold by mutant A delta 5, whereas mRNA synthesis directed by the actin promoter was not affected. Furthermore, virus production in COS, HeLa, and Jurkat T cells was inhibited 45-, 5-, and 3-fold, respectively, by mutant A delta 5. These results demonstrate that the balance between PP2A holoenzyme and core enzyme is important for HIV-1 gene expression and virus production.

Molecular genetic analysis of Rts1p, a B' regulatory subunit of Saccharomyces cerevisiae protein phosphatase 2A

The Saccharomyces cerevisiae gene RTS1 encodes a protein homologous to a variable B-type regulatory subunit of the mammalian heterotrimeric serine/threonine protein phosphatase 2A (PP2A). We present evidence showing that Rts1p assembles into similar heterotrimeric complexes in yeast. Strains in which RTS1 has been disrupted are temperature sensitive (ts) for growth, are hypersensitive to ethanol, are unable to grow with glycerol as their only carbon source, and accumulate at nonpermissive temperatures predominantly as large-budded cells with a 2N DNA content and a nondivided nucleus. This cell cycle arrest can be overcome and partial suppression of the ts phenotype of rts1-null cells occurs if the gene CLB2, encoding a Cdc28 kinase-associated B-type cyclin, is expressed on a high-copy-number plasmid. However, CLB2 overexpression has no suppressive effects on other aspects of the rts1-null phenotype. Expression of truncated forms of Rts1p can also partially suppress the ts phenotype and can fully suppress the inability of cells to grow on glycerol and the hypersensitivity of cells to ethanol. By contrast, the truncated forms do not suppress the accumulation of large-budded cells at high temperatures. Coexpression of truncated Rts1p and high levels of Clb2p fully suppresses the ts phenotype, indicating that the inhibition of growth of rts1-null cells at high temperatures is due to both stress-related and cell cycle-related defects. Genetic analyses show that the role played by Rts1p in PP2A regulation is distinctly different from that played by the other known variable B regulatory subunit, Cdc55p, a protein recently implicated in checkpoint control regulation.

Molecular heterogeneity of the cDNA encoding a 74-kDa regulatory subunit (B" or delta) of human protein phosphatase 2A

Two cDNAs for possible splicing variants of a 74-kDa regulatory subunit (B" or delta) of human protein phosphatase 2A, were isolated. These variants were identified from human cerebral cortex by library screening and PCR, and designated delta1 and delta3 isoforms, while the previously reported isoform [Tanabe et al. (1996) FEBS Lett. 379, 107-1111 was designated delta2. Compared with the delta2 isoform, the delta1 isoform contained a 32-residue insertion beginning at residue 84, and consisted of 602 amino acids in all. The delta3 isoform lacked a 74-residue sequence corresponding to residues 1083 of the delta2 isoform, and consisted of 496 amino acids. Using isoform-specific antipeptide antisera, the 74-kDa subunit (B" or delta) originally purified from human erythrocytes was identified as the delta1 isoform.

Differential expression of three Arabidopsis genes encoding the B' regulatory subunit of protein phosphatase 2A

Numerous plant processes ranging from signal transduction to metabolism appear to be mediated, in part, by type 2A protein serine/threonine phosphatases (PP2A). In an effort to identify factors that control the activity of this enzyme in plants, we have isolated and characterized DNA sequences encoding the B' regulatory subunit of PP2A from Arabidopsis thaliana. Specifically, we used PCR to amplify a segment of Arabidopsis cDNA that encodes a conserved section of the B' polypeptide. This PCR fragment was subsequently used as a probe to screen an Arabidopsis cDNA library and cDNA clones derived from three distinct genes were identified. The AtB' alpha and AtB' beta genes encode highly similar 57-kDa B' regulatory subunits while the third gene, AtB' gamma, encodes a more divergent 59-kDa B' protein. A comparison of the three Arabidopsis B' polypeptides to those of yeast and animals shows the core region of this protein to be the most conserved while the amino and carboxy termini vary both in length and sequence. Genomic Southern blots indicate that at most the Arabidopsis genome contains five genes encoding the B' regulatory subunit. The three genes identified in this study are expressed in all Arabidopsis organs, albeit at varying levels. In addition, mRNAs derived from the three genes accumulate differentially in response to heat shock. Our results indicate that the activity of plant PP2A might be regulated by a B' type regulatory subunit similar to those found in animals and yeast, and suggest possible roles for B'-containing PP2A complexes within plant cells.

Saccharomyces cerevisiae homologs of mammalian B and B' subunits of protein phosphatase 2A direct the enzyme to distinct cellular functions

Protein phosphatase 2A (PP2A) is a major cellular serine/threonine protein phosphatase, present in the cell in a variety of heterotrimeric forms that differ in their associated regulatory B-subunit. Cloning of the mammalian B' subunit has allowed the identification of a highly homologous Saccharomyces cerevisiae gene, RTS1. Disruption of the gene results in a temperature-sensitive growth defect that can be suppressed by expression of rabbit B'alpha or B'gamma isoforms. The B'alpha subunit is much more effective in restoring normal growth at 37 degrees C than B'gamma. Immunoprecipitated Rts1p was found associated with type 2A-specific protein phosphatase activity that is sensitive to 2 nM okadaic acid, but not to 100 nM phosphatase inhibitor-2, and to be phosphorylated in vivo. However, overexpression of RTS1 was unable to suppress the cold sensitivity, defective cytokinesis, and abnormal cell morphology resulting from defects in the CDC55 gene, which encodes the yeast homolog of a different B subunit of another form of 2A phosphatase, PP2A1. These results indicate that Rts1p is a yeast homolog of the mammalian B' subunit and that the various regulatory B-subunits of PP2A are not functionally redundant but direct the enzyme to distinct cellular functions.

Separation of PP2A core enzyme and holoenzyme with monoclonal antibodies against the regulatory A subunit: abundant expression of both forms in cells

Protein phosphatase 2A (PP2A) holoenzyme is composed of a catalytic subunit, C, and two regulatory subunits, A and B. The A subunit is rod shaped and consists of 15 nonidentical repeats. According to our previous model, the B subunit binds to repeats 1 through 10 and the C subunit binds to repeats 11 through 15 of the A subunit. Another form of PP2A, core enzyme, is composed only of subunits A and C. It is generally believed that core enzyme does not exist in cells but is an artifact of enzyme purification. To study the structure and relative abundance of different forms of PP2A, we generated monoclonal antibodies against the native A subunit. Two antibodies, 5H4 and 1A12, recognized epitopes in repeat 1 near the N terminus and immunoprecipitated free A subunit and core enzyme but not holoenzyme. Another antibody, 6G3, recognized an epitope in repeat 15 at the C terminus and precipitated only the free A subunit. Monoclonal antibodies against a peptide corresponding to the N-terminal 11 amino acids of the A alpha subunit (designated 6F9) precipitated free A subunit, core enzyme, and holoenzyme. 6F9, but not 5H4, recognized holoenzymes containing either B, B', or B" subunits. These results demonstrate that B subunits from three unrelated gene families all bind to repeat 1 of the A subunit, and the results confirm and extend our model of the holoenzyme. By sequential immunoprecipitations with 5H4 or 1A12 followed by 6F9, core enzyme and holoenzyme in cytoplasmic extracts from 10T1/2 cells were completely separated and they exhibited the expected specificities towards phosphorylase a and retinoblastoma peptide as substrates. Quantitative analysis showed that under conditions which minimized proteolysis and dissociation of holoenzyme, core enzyme represented at least one-third of the total PP2A. We conclude that core enzyme is an abundant form in cells rather than an artifact of isolation. The biological implications of this finding are discussed.

p53-dependent association between cyclin G and the B' subunit of protein phosphatase 2A

We and others previously showed that cyclin G is a transcriptional target of the p53 tumor suppressor protein. However, cellular proteins which might form a complex with cyclin G have not yet been identified. To gain insight into the biological role of cyclin G, we used the yeast two-hybrid screen and isolated two mouse cDNAs encoding cyclin G-interacting proteins. Interestingly, both positive cDNAs encoded B' regulatory subunits of protein phosphatase 2A (PP2A). One clone encodes B'alpha, while the other clone codes for a new member of the B' family, B'beta. B'beta is 70% identical to other members of the B' family. B'alpha associated both in vitro and in vivo with cyclin G but not with the other mammalian cyclins. Furthermore, cyclin G formed a complex with B'alpha only after induction of p53 in p53 temperature-sensitive cell lines. These results indicate that cyclin G forms a specific complex with the B' subunit of PP2A and that complex formation is regulated by p53. Potential roles for the cyclin G-B' complex in p53-mediated pathways are discussed.

The B56 family of protein phosphatase 2A (PP2A) regulatory subunits encodes differentiation-induced phosphoproteins that target PP2A to both nucleus and cytoplasm

Protein phosphatase 2A is a heterotrimeric protein serine/threonine phosphatase consisting of a 36-kDa catalytic C subunit, a 65-kDa structural A subunit, and a variable regulatory B subunit. The B subunits determine the substrate specificity of the enzyme. There have been three families of cellular B subunits identified to date: B55, B56 (B'), and PR72/130. We have now cloned five genes encoding human B56 isoforms. Polypeptides encoded by all but one splice variant (B56gamma1) are phosphoproteins, as shown by mobility shift after treatment with alkaline phosphatase and metabolic labeling with [32P]phosphate. All labeled isoforms contain solely phosphoserine. Indirect immunofluorescence microscopy demonstrates distinct patterns of intracellular targeting by different B56 isoforms. Specifically, B56alpha, B56beta, and B56epsilon complexed with the protein phosphatase 2A A and C subunits localize to the cytoplasm, whereas B56delta, B56gamma1, and B56gamma3 are concentrated in the nucleus. Two isoforms (B56beta and B56delta) are highly expressed in adult brain; here we show that mRNA for these isoforms increases severalfold when neuroblastoma cell lines are induced to differentiate by retinoic acid treatment. These studies demonstrate an increasing diversity of regulatory mechanisms to control the activity of this key intracellular protein phosphatase and suggest distinct functions for isoforms targeted to different intracellular locations.

Protein serine/threonine phosphatases

In the past year, the three-dimensional structures of two serine/threonine phosphatases, protein phosphatase-1 and protein phosphatase-2b (calcineurin), have been determined. The new information puts previous sequence comparisons and mutagenesis studies into a detailed structural perspective. The active-site structure and catalytic mechanism appear to be common to a variety of phosphoesterase enzymes.

The variable subunit associated with protein phosphatase 2A0 defines a novel multimember family of regulatory subunits

Two protein phosphatase 2A (PP2A) holoenzymes were isolated from rabbit skeletal muscle containing, in addition to the catalytic and PR65 regulatory subunits, proteins of apparent molecular masses of 61 and 56 kDa respectively. Both holoenzymes displayed low basal phosphorylase phosphatase activity, which could be stimulated by protamine to an extent similar to that of previously characterized PP2A holoenzymes. Protein micro-sequencing of tryptic peptides derived from the 61 kDa protein, termed PR61, yielded 117 residues of amino acid sequence. Molecular cloning by enrichment of specific mRNAs, followed by reverse transcription-PCR and cDNA library screening, revealed that this protein exists in multiple isoforms encoded by at least three genes, one of which gives rise to several splicing variants. Comparisons of these sequences with the available databases identified one more human gene and predicted another based on a rabbit cDNA-derived sequence, thus bringing the number of genes encoding PR61 family members to five. Peptide sequences derived from PR61 corresponded to the deduced amino acid sequences of either alpha or beta isoforms, indicating that the purified PP2A preparation was a mixture of at least two trimers. In contrast, the 56 kDa subunit (termed PR56) seems to correspond to the epsilon isoform of PR61. Several regulatory subunits of PP2A belonging to the PR61 family contain consensus sequences for nuclear localization and might therefore target PP2A to nuclear substrates.

Molecular identification of I1PP2A, a novel potent heat-stable inhibitor protein of protein phosphatase 2A

The amino acid sequences of two tryptic peptides derived from purified preparations of I1PP2A indicated that this potent heat-stable protein inhibitor of protein phosphatase 2A (PP2A) may be equivalent to putative histocompatibility leukocyte antigens class II-associated protein I (PHAP-I). Experiments using purified preparations of recombinant human PHAP-I confirmed that this protein inhibited PP2A. Half-maximal inhibition of the phosphatase occurred at about 4 nM PHAP-I, similar to the half-maximal inhibition obtained with purified preparations of bovine kidney I1PP2A. In addition, PHAP-I did not affect the activities of protein phosphatase 1, 2B, and 2C in a manner analogous to that of I1PP2A. Together, the results establish the identity of I1PP2A on a firm basis.

The myeloid leukemia-associated protein SET is a potent inhibitor of protein phosphatase 2A

Two potent heat-stable protein phosphatase 2A (PP2A) inhibitor proteins designated I1PP2A and I2PP2A have been purified to apparent homogeneity from extracts of bovine kidney (Li, M., Guo, H., and Damuni, Z. (1995) Biochemistry 34, 1988-1996). N-terminal and internal amino acid sequencing indicated that I2PP2A was a truncated form of SET, a largely nuclear protein that is fused to nucleoporin Nup214 in acute non-lymphocytic myeloid leukemia. Experiments using purified preparations of recombinant human SET confirmed that this protein inhibited PP2A. Half-maximal inhibition of the phosphatase occurred at about 2 nM SET. By contrast, SET (up to 20 nM) did not affect the activities of purified preparations of protein phosphatases 1, 2B, and 2C. The results indicate that SET is a potent and specific inhibitor of PP2A and suggest that impaired regulation of PP2A may contribute to acute myeloid leukemogenesis.

Identification of a novel protein phosphatase 2A regulatory subunit highly expressed in muscle

Differential association of regulatory B subunits with a core heterodimer, composed of a catalytic (C) and a structural (A) subunit, is an important mechanism that regulates protein phosphatase 2A (PP2A). We have isolated and characterized three novel cDNAs related to the B' subunit of bovine cardiac PP2A. Two human (B'alpha1 and B'alpha2) and a mouse (B'alpha3) cDNA encode for alternatively spliced variants of the B subunit. The deduced primary sequences of these clones contain 12 of 15 peptides derived from the purified bovine B' subunit. Differences between the deduced sequences of the B alpha splice variants and the cardiac peptide sequences suggest the existence of multiple isoforms of the B' subunit. Comparison of the protein and nucleotide sequences of the cloned cDNAs show that all three forms of B'alpha diverge at a common splice site near the 3'-end of the coding regions. Northern blot and reverse transcription-polymerase chain reaction analyses revealed that the B'alpha transcripts (4.3-4.4 kb) are widely expressed and very abundant in heart and skeletal muscle. The expressed human and mouse B'alpha proteins readily associated with the PP2A core enzyme in both in vitro and in vivo complex formation assays. Immunofluorescence microscopy revealed that epitope-tagged B'alpha was localized in both the cytosol and nuclei of transiently transfected cells. The efficiency of binding of all three expressed proteins to a glutathione S-transferase-A subunit fusion protein was greatly enhanced by the addition of the C subunit. Expression of the B'alpha subunits in insect Sf9 cells resulted in formation of AC.B'alpha heterotrimers with the endogenous insect A and C subunits. These results show that the B' subunit, which is the predominant regulatory subunit in cardiac PP2A, is a novel protein whose sequence is unrelated to other PP2A regulatory subunits. The nuclear localization of expressed B'alpha suggests that some variants of the B' subunit are involved in the nuclear functions of PP2A.

Molecular cloning of a 74-kDa regulatory subunit (B" or delta) of human protein phosphatase 2A

Based on amino acid sequence data of a 74-kDa regulatory subunit (B" or delta) of a human heterotrimeric protein phosphatase 2A, a cDNA encoding the subunit was isolated from a human cerebral cortex library. The cDNA had an open reading frame encoding an M(r) 66,138 protein of 570 amino acids. Bacterial expression of the cDNA yielded a protein immunoreactive with antisera specific to the 74-kDa subunit. The predicted primary structure of the subunit had no similarity to already reported sequences of PP2A regulatory subunits including A, B, and PR72. Potential phosphorylation sites for protein kinases A and C, a bipartite motif of putative nuclear localization signal, and SH3 accessible proline-rich domain, and a unique PQ repeat were found in the sequence. The subunit mRNA of about 2.9 kb was ubiquitously expressed in rat tissues.